1. Field of the Invention
The present invention relates to a driving circuit which drives light-emitting diodes, and in particular to a light-emitting diode driving apparatus which drives light-emitting diodes by using AC power supply.
2. Description of the Related Art
In recent years, significant attention is given to light-emitting diodes (hereinafter, occasionally referred to as “LEDs”) as lighting sources. The reason is that LEDs can be driven at low power consumption as compared with filament lamps or fluorescent lamps. LEDs are small, and have shock resistance. In addition, LEDs are less prone to blow out. Thus, LEDs have these advantages.
In the case of lighting sources, it is desirable that commercial AC power for home use is used as power supply for lighting sources. LEDs are devices driven by DC power. LEDs emit light only when applied with a current in the forward direction. Also, in the case of LEDs that are currently typically used for lighting use, the LEDs operate on DC power at a forward directional voltage Vf of about 3.5 V. LEDs do not emit light if a voltage applied to the LEDs does not reach Vf. Conversely, a voltage applied to the LEDs exceeds Vf, an excessive amount of current will flow through the LEDs. Accordingly, it can be said that DC power is suitable for driving LEDs.
To satisfy the contradictory conditions, various types of LED driving circuits have been proposed that use AC power. For example, a method has been proposed that switches LEDs so that a Vf total value is changed in accordance with a varying voltage value (see Japanese Patent Laid-Open Publication No. JP 2006-147,933 A). In this method, a number of LEDs connected to each other in series are divided into blocks 161, 162, 163, 164, 165 and 166 as shown in a circuit diagram of FIG. 6. The LED blocks 161 to 166 are selectively connected to the power supply in accordance with the voltage value of input voltage of rectified waveform by a switch control portion 167 consisting of a microcomputer so that a Vf total value is changed in a stepped manner. As a result, as shown by a voltage waveform in a timing chart of FIG. 7, since the LEDs can be driven by a plurality of rectangular waves corresponding to the rectified waveform, the LED usage efficiency can be improved as compared with the ON-duty in the case of only single rectangular wave.
On the other hand, the applicant has been developed an AC multi-stage circuit which includes a plurality of serially-connected LED blocks operated by an AC current after full-wave rectification, each of the plurality of LED blocks having a plurality of serially-connected LEDs (Japanese Patent Laid-Open Publication No. JP 2011-40,701 A).
As shown in FIG. 8, this AC multi-stage circuit subjects a current from an AC power supply AP to full-wave rectification in a bridge circuit 2 so that the LED blocks of multi stages are supplied with the current after the full-wave rectification. As the LED blocks of multi stages, first, second and third LED blocks 11, 12 and 13 are serially connected to each other. A first LED current control transistor 21A is turned ON/OFF to connect/disconnect a first bypass BP1 which bypasses the second LED block 12 based on the current amount in the first LED block 11. A second LED current control transistor 22A is turned ON/OFF to connect/disconnect a second bypass BP2 which bypasses the third LED block 13 based on the current amount in the first and second LED blocks 11 and 12. This AC multi-stage circuit can keep power supply efficiency high, and additionally improve the LED usage efficiency and the power factor.
The applicant has been developed a light-emitting diode driving apparatus which includes a plurality of LEDs connected to each other and can suppress harmonic components as shown in FIG. 9. FIG. 10 is a graph showing a current waveform obtained by this light-emitting diode driving apparatus. Thus, harmonic distortion can be suppressed so that the LEDs can be driven by a current with the current waveform close to sine wave.
On the other hand, in the case where conventional filament lamps are used as light emitting elements instead of LEDs, a current flowing in the filament lamps will also have a substantially sine waveform. In the case of filament lamps, since light produced as incandescence from filament, the light does not fluctuate at power supply frequency (50 Hz or 60 Hz), in other words, flicker does not occur. Contrary to this, in the case where LEDs are used as light emitting elements, since LEDs have high responsivity, there is a problem that flicker will occur at the power supply frequency. This can be seen from a light output waveform of a sine wave multi-stage driving circuit shown in FIG. 11. The crest factor (=maximum value/effective value) is used as objective index, and gets better as it gets closer to 1 (one). The calculated crest factor of this light output shown in FIG. 11 is not smaller than 1.5. This value is worse than filament lamp of about 1.05, fluorescent lamp of about 1.36, and inverted fluorescent lamp of about 1.1. This means that some people may perceive flicker. In case where the LEDs are lighting a rotating body, if the power supply frequency matches with the rotating frequency, it may perceived that the rotating body is stopped even though it rotates. Accordingly, these may cause poor lighting quality. For this reason, if the light-emitting diode driving apparatus shown in FIG. 9 is used for high quality lighting, it is necessary to eliminate the light OFF period and to improve its crest factor.
It can be conceived that a capacitor is used for smoothing to eliminate flicker. That is, it can be conceived that the capacitor is charged in the period where a power supply voltage is high, and is discharged in the period where the voltage is low. However, if a capacitor is used, the capacitor will be rapidly charged in a short charging period, which in turn increases the charging current. In addition, the charging current will be increased with the capacity of the capacitor. In the case of a large capacitor to be used for such smoothing, the charging current will be further increased, which in turn deteriorates the power factor. For this reason, such a light-emitting diode driving apparatus may not meet the harmonic distortion standards. On the other hand, it can be conceived that an active filter IC is additionally used to improve the power factor. However, there are disadvantages that such an element is expensive, and that high frequency switching noise may be produced.
The present invention is devised to solve the above problems. It is a main object of the present invention is to provide a light-emitting diode driving apparatus which can improve its crest factor by reducing the light OFF period without distorting an input current waveform approximating a sine wave.